1. Field of the Invention
The present invention relates to a method and an apparatus for continuously forming deposited films on an elongated substrate by a plasma CVD method. More particularly, the present invention relates to a method and an apparatus for forming deposited films suitable to mass-produce devices, such as solar cells or sensors, that are photoelectric conversion devices using an amorphous semiconductor which is a non-single-crystalline (non-monocrystalline) semiconductor, or a microcrystal semiconductor.
2. Related Background Art
In recent years, consumption of electric power has rapidly increased all over the world and electric power production has increased correspondingly. However, it leads to the problems that thermal power generation and nuclear electric power generation cause environmental pollution and global warming. On the other hand, solar cell power generation using sunlight does not raise the foregoing problems of environmental pollution, global warming, and maldistribution of resources. Therefore, solar cell power generation is worthy of consideration for meeting a further large requirement for electric power.
To realize large scale solar cell power generation, solar cells must have a satisfactory photoelectrical conversion efficiency and stable characteristics while being suitable to mass production so as to be supplied cheaply. Furthermore, solar cells having a large area are required because of the large scale of the required power generation. Therefore, it has been proposed that an amorphous silicon type solar cell be formed by depositing a semiconductor thin film, such as of amorphous silicon, which is a non-single-crystal material, on a relatively low cost substrate, such as glass or a metal sheet, by decomposing a readily available raw material gas of silane or the like by glow discharge. The amorphous silicon type solar cell exhibits excellent mass productivity and lower cost as compared with a solar cell manufactured from single-crystal silicon, thus attracting attention. A variety of manufacturing methods have thus far been proposed.
The solar cell power generation system usually has a structure wherein modules formed of a plurality of solar cells, are connected in series or in parallel so that a desired electric current and voltage are obtained. Therefore, disconnections and short circuits in each unit module must be prevented. Furthermore, nonuniformity of the output voltage and electric currents of the unit modules must be prevented. To meet these requirements, the characteristics of the semiconductor, which are the most important factor determining the characteristics of the module, must have uniformity. To simplify the process for assembling the module, a deposited film of a semiconductor must be formed over a large area and must have excellent characteristics. The mass productivity of the solar cell thereby can be improved and the manufacturing cost can be significantly reduced.
The semiconductor, which is the most important component of the solar cell, includes semiconductor junctions, such as p-n junctions, p-i-n junctions or the like. The foregoing semiconductor junctions can be formed by sequentially depositing semiconductors having different conductivity types, or by implanting into a semiconductor having a certain conductivity type dopants having a different conductivity type or by thermal diffusion of dopants. When an amorphous silicon solar cell is manufactured, the following known method is employed which includes the steps of mixing a raw material gas containing elements as dopants such as phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6) or the like with a silane gas or the like which is the main raw material gas; decomposing the mixed raw material gases by glow discharge to form semiconductor films having desired conductivity types; and sequentially depositing the thus-obtained semiconductor films on a desired substrate to form a semiconductor junction. Therefore, when the amorphous silicon solar cell is manufactured, interdiffusion of dopants having different conductivity types is generally prevented by providing independent film forming chambers corresponding to the semiconductor layers to be formed.
As a method of forming a deposited film by a plasma CVD method suitable to manufacture an amorphous silicon solar cell of the foregoing type, a roll-to-roll method has been disclosed in U.S. Pat. No. 4,400,409. The foregoing film forming method comprises the steps of disposing a plurality of glow discharge regions along the path of an elongated (belt-like) substrate, and continuously conversing the elongated substrate in its longitudinal direction while forming semiconductor layers respectively having desired conductivity types in the corresponding glow discharge regions. According to the above patent, solar cells having desired semiconductor junctions can be formed continuously. The foregoing method of forming deposited films has a structure for preventing interdiffusion and mixture of the dopant gas for use in each glow discharge region into other glow discharge regions, the structure being arranged such that the respective glow discharge regions are mutually separated from one another by slit-like separation passages called gas gates and that gas flows of, for example Ar or H.sub.2, are formed to flow through the separation passages. As a result of employing the above structure, the roll-to-roll method for forming a deposited film is suitable to manufacture the semiconductor layers for the solar cell or the like.
On the other hand, it is known that the photoelectric conversion efficiency of the amorphous silicon solar cell can be improved when a Group IV alloy semiconductor, such as a-SiGe:H, a-SiGe:F, a-SiGe:H:F, a-SiC:H, a-SiC:F or a-SiC:H:F is used as the i-type (intrinsic type) semiconductor by continuously changing the forbidden band gap (Egopt) of the i-type semiconductor in the direction of the thickness from the light incident side to improve the open circuit voltage (Voc) and fill factor (FF) of the solar cell (20th IEEE PVSC, "A Novel Design for Amorphous Silicon Solar Cells", S. Guha, J. Yang, et al.).
Hitherto, the process for continuously forming a deposited film having a large area involves diffusion of impurities into the i-type semiconductor due to an increase in the substrate temperature and thus the characteristics of the solar cell are deteriorated. In order to overcome the problem above, a method has been employed in which the substrate temperature is sequentially lowered as the respective semiconductor layers are deposited during the step of depositing the films. However, the conventional method cannot form a deposited film at a substrate temperature suitable for each deposited film. Therefore, the characteristics of the deposited film are not satisfactory. Therefore, a problem has occurred in that the characteristics of the photoelectric conversion device formed by depositing a plurality of layers have been unsatisfactory.
The foregoing roll-to-roll method for forming a deposited film has an arrangement wherein the elongated substrate is continuously moved while forming the film. Therefore, improvement in the manufacturing efficiency requires the moving speed of the elongated substrate and the film deposition speed to be increased. However, increasing the speeds leads to deterioration of the characteristics of the photoelectric conversion device.
The film quality of the semiconductor layer having the i-type conductivity must be improved when the characteristics of the photoelectric conversion device are to be improved. However, additional electric power is required to increase the substrate temperature or the bias electric power.
However, the additional electric power raises the substrate temperature excessively, and impurities are undesirably diffused into the i-type semiconductor due to annealing of the deposited films. Therefore, it is difficult to improve the characteristics of the photoelectric conversion device by the foregoing roll-to-toll method.
As disclosed in the U.S. Pat. Nos. 4,389,970 and 4,470,369, the foregoing roll-to-roll method for forming a deposited film controls the substrate temperature at a predetermined level during formation of the deposited film. Therefore, it does not include the idea of changing the substrate temperature at a predetermined rate before and after forming a deposited film so as to prevent annealing of the deposited film.